The present invention relates generally to superconductive magnets and more particularly to a superconducting-magnet electrical circuit which protects its superconductive-coil assemblage from damage during a quench.
Superconducting magnets are those superconducting devices which have a superconductive-coil assemblage and include, but are not limited to, magnetic resonance imaging (MRI) systems for medical diagnosis, superconductive rotors for electric generators and motors, and magnetic levitation devices for train transportation. Magnets wound of superconductive material offer the advantage that they can carry significant electrical currents without suffering any power loss due to the fact that superconductive windings offer no resistance to electrical current flow. As a consequence of this zero resistance property, wire or tape that is quite small is capable of carrying very large currents in the superconducting state. This property has been especially beneficial in the construction of MRI magnets because they require very high static magnetic fields.
When designing superconducting magnets, however, one must consider the possibility that the superconducting wire or tape may "lose" its superconducting capabilities at some point and become resistive. This transformation from a superconducting state to a resistive state is known as "quenching" and may be caused, for example, by loss of the cryogenic cooling needed for superconductivity to occur. In the event that the superconductive wire becomes resistive, the wire that normally carries the large electrical currents with no resistive heating now generates both high voltages and high power losses. These voltages and power losses can be quite damaging to the magnet if they are allowed to become too large or remain too localized.
As a consequence of the above, magnets are designed such that the "quench" is propagated as quickly as possible after initiation; that is, if some area of the winding quenches, the magnet is designed so that the entire winding becomes resistive as soon as possible. This design criteria results in lower voltages and lower peak temperatures since the stored energy of the magnet is dispersed throughout a larger mass. Known quench protection techniques include using a quench-detection signal (from the electrical center of the superconductive coil assemblage of the superconductive device) directly supplying an energy dump resistor or directly powering a wide-area electrical heater located near the superconductive coil assemblage of the superconductive device. Such known techniques take a relatively long time to work. It is also known to amplify the quench-detection signal outside the cryostat, but this raises issues of reliability and additional cryostat penetrations.